1. Technical Field
The invention relates generally to polypropylene and materials or articles made from polypropylene.
2. Background of the Art
Polypropylene is used for a variety of different products or applications. These may include films, fibers or molded articles. Polypropylene used in such materials or articles is usually produced as an isotactic propylene polymer, which is a stereospecific polymer.
Stereospecific polymers are polymers that have a defined arrangement of molecules in space. Both isotactic and syndiotactic propylene polymers are stereospecific. Isotactic polypropylene is characterized by having all the pendant methyl groups oriented either above or below the polymer chain or backbone. Isotactic polypropylene can be illustrated by the following general chemical formula:

Syndiotactic propylene polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the polymer chain lie on alternate sides of the plane of the polymer. Syndiotactic polypropylene can be illustrated by the following general structural formula:

While both syndiotactic and isotactic polypropylene are semi-crystalline polymers, however, they each have different characteristics or properties.
Conventional polypropylene is usually prepared as an isotactic polymer from Ziegler-Natta polymer catalysts. The Ziegler-Natta catalysts produce a highly isotactic polypropylene that is easily processed and useful in preparing a wide variety of articles or products.
In certain applications, it is necessary that the polypropylene materials be sterilized. This is particularly true for materials used in medical and food handling and sterilization applications. One method of sterilizing such materials is through the use of high-energy radiation. Both gamma radiation and electron-beam (E-beam) radiation are commonly used for irradiating and sterilizing many materials and articles. While exposure to such radiation is effective in sterilizing such materials, the radiation may also have an effect on the material itself. In many cases, these effects are undesirable.
With respect to isotactic polypropylene prepared from conventional Ziegler-Natta catalysts, for example, exposure of the polypropylene to high-energy radiation can result in a degradation of the polymer. The polypropylene will often become brittle and may be discolored, turning to a light or deep yellow. Such changes in the polymer usually do not occur immediately after irradiation, but may occur slowly, appearing sometime later after sterilization.
The mechanism by which such degradation of polypropylene occurs is believed to be, without being limited to any one particular theory, an auto-oxidative reaction in which free radicals are formed that react with oxygen, usually from air, and which results in the degradation of the polymer. The reaction steps can be represented as follows:R→R.  (1)R.+O2→RO2.  (2)RO2.+RH→ROOH+R.  (3)RO2.+R.→ROOR  (4)RO2.+RO2.→ROOR+O2  (5)R.+R.→R−R  (6)where R is the irradiated polypropylene chain, and R. is the alkyl radical formed during irradiation. The alkyl radical R. is regenerated in equation 3 and each alkyl radical formed will consume numerous molecules of oxygen unless such radicals are terminated earlier as shown in equations 4-6.
As discussed earlier, degradation effects are usually seen over time. This may be a result, at least in part, due to slower radical migration from within the crystalline regions of the polymer towards the surface to react with ambient oxygen. Thus, polymer degradation may occur over time as a result of this radical migration. Polypropylene articles having high surface areas per unit volume will usually tend to degrade much faster than those having low surface areas per unit volume.